MDL-43266 testing: avoid assoc arrays for json

Last commit switched from raw arrays to associative
arrays. That leads to json_encode() to generate an
object instead of an array, ultimately leading to
problems with the rest of code expecting an array.

By using array_values() we are moving back to the
non-associative array.

Also, fix some file permissions.

lib/testing/classes/util.php

@@ -879,7 +879,7 @@ abstract class testing_util {
 
             // Save the file list in a JSON file.
             $fp = fopen($jsonfilepath, 'w');
-            fwrite($fp, json_encode($listfiles));
+            fwrite($fp, json_encode(array_values($listfiles)));
             fclose($fp);
         }
     }